In the art various proposals have been made to achieve effective printing on substrates by causing a change of color in the substrate on which the printing is to appear. Such printing may result in marking, such as an image, a letter, a figure, a text, etc. As an example, a sell-by-date may be printed on prepack coated with marking components, i.e. activatable pigments.
Various marking compounds, i.e. activatable pigments, have been proposed, which may be used to mark a substrate upon irradiation, such as by laser. As an example, WO 02/01250 discloses the use of various oxymetal salts, such as ammonium octamolybdate (AOM), in laser marking applications.
However, as recognized in the art, laser markable aqueous compositions based on AOM suffer from giving rise to an opaque coating before activation. In addition the reactivity of AOM is, for some applications, deemed to be to low.